Within the past decade, the telecommunication industry has seen a marked increase in demand not only for globally interconnected telephone services but also for global interconnection for broadband services. The demand for such services has traditionally been met (at least in part) by some combination of terrestrial facilities, including cellular communication systems. However, attempting to satisfy the ever increasing demand for these services by adding additional terrestrial facilities would be very costly and time consuming. An alternative possible solution to satisfy the increasing demand for these services would be to employ large Geosynchronous Earth Orbit (GEO) satellites. However, this solution alone would be extremely costly, and impractical for providing service to customer terminals (whether fixed or mobile) because extremely high power would be required to communicate with satellites operating at such a high altitude.
Thus, in recent years, the industry has seen the first serious consideration of Nongeostationary Orbit (NGSO) satellite constellations using Low Earth Orbit (LEO) and/or Medium Earth Orbit (MEO) satellites and/or some combination of LEO, MEO, and/or GEO satellites to respond to the rapidly expanding demand for global telecommunication services. Such satellite communication systems have the potential to provide world-wide and/or regional coverage at a much lower cost than would be possible using a terrestrial network. Such satellite communication systems also have the potential for providing economical services to virtually any point on the earth through satellite-to-satellite and satellite-to-ground links, even to remote or sparsely populated areas where it may not be economically feasible to deploy a terrestrial network. Of various satellite communication systems that have been proposed, some propose to provide world-wide coverage, while others propose to provide regional coverage, or coverage within a particular latitude band. Among these satellite communication systems, use of satellite constellations designed with both polar and inclined orbits have been proposed.
All of these satellite communication systems must address the problem of meeting high performance standards and capacity requirements in a cost-effective manner. Meeting the demand to provide readily available, high capacity service in all coverage areas of a satellite communication system in a financially feasible manner can be extremely difficult. Several major network operational issues complicate the ability to provide cost-effective, high quality service through a satellite communication network or system.
First, various geographical service regions within the coverage area of the satellite communication system could have differing demands in terms of level of capacity required to service the area. Similarly, specific locations within a geographical service region also could have varying demands on system capacity, and these demands could vary even more by time of day (e.g., during business hours or other peak communication times). Additionally, demands from various regions and locations can change over time as certain regions become more or less populated. Designing each satellite in a satellite communication system to be flexible and capable of handling the maximum possible load from any service region or location within the coverage area would be cost prohibitive and would result in very large system components, including satellites. Satellite constellations comprised of very large satellites, and/or extremely large numbers of smaller satellites, would be financially unrealistic.
Second, satellite communication systems must address the problem of how to provide continuing service in all coverage areas in the event of a failure of one or more satellites in the satellite constellation. Even a single satellite failure likely will not be tolerated by satellite communication system users having high expectations, and such a satellite failure could have grave repercussions on the satellite communication system provider not only in terms of lost revenue, but in terms of loss of future revenues from negative effects on the reputation of the provider.
Known failure mitigation techniques include, for example, launching replacement satellites or moving spare satellites into the orbital position of the failed satellite. However these solutions are not optimal as they are very costly and can take days or even weeks to implement. Meanwhile, revenue is lost until the failed satellite is replaced.
A third problem faced by satellite communication systems is the need to dynamically maintain service links to satellite communication system users using equipment that can be situated either on or below the surface of the earth or in the atmosphere above the earth, and can be either mobile or fixed. A satellite communication system projects a number of "beams" or "cells" over the earth. A NGSO mobile satellite communication system must address the motion of satellite antenna beam coverage areas (hereinafter referred to as "satellite footprints") relative to the a communication unit (hereinafter referred to as "CU") used by a satellite communication system user to communicate through the satellite communication system. As satellites sweep over the earth's surface, a given CU may be handed-off not only between multiple beams of a single satellite, but also between two or more different satellites during the course of the communication.
Some prior art satellite communication systems accommodate satellite-to-satellite hand-offs by providing a small overlap between satellite footprints. These satellite communication systems generally provide only enough overlapping coverage to insure that hand-offs from a setting satellite (moving away from the CU) to a rising satellite (moving toward the CU) occur near the edge of a footprint. Rise and set geometry is defined by the design minimum elevation angle. This angle, when combined with the altitude of the satellite constellation, defines the size of the satellite footprint needed to cover the service area. Prior art satellite communication systems have little control over the service elevation angle at which the CU communicates with a satellite; rather, constellation dynamics and the local environment dictate the elevation angle for service link maintenance.
Other prior art satellite communication systems have used multiple satellite coverage to maintain service links during a communication and to simplify satellite-to-satellite hand-offs by having more than one satellite at time communicate with each CU using the same channel. The CU then combines all of the signals it receives from the multiple satellites to bolster the link margin and maintain the service link through hand-offs. Such a satellite communication system requires complete double satellite coverage, however, and is very costly.
A fourth network operational issue that must be addressed by satellite communication systems is the issue of service blockage due either to physical obstacles, such as buildings and trees, or weather obstacles, such as precipitation, or other similar obstacles situated within the CU's line of view to a satellite. Prior art satellite communication systems, and LEO satellite communication systems in particular have little or no resources available to mitigate the effects of blockage of access of a particular CU to the satellite communication system occurring when the line of view between the CU and a satellite is obstructed by an obstacle. This is because each CU generally views only one satellite at a time or views multiple satellites from the same elevation angle. Thus, if the line of view between the CU and a satellite is obstructed, there may be no alternative satellite with which the CU might communicate.
Thus, what is needed is a method and apparatus to manage resources in a satellite communication system so as to mitigate the effects of network operational issues. What is further needed is a method and apparatus for providing an economically viable satellite communication system for voice, data, and video using, at least in some significant part, relatively inexpensive satellites. What is further needed is a method and apparatus having flexibility to adapt to changing demand needs in regions being serviced by a satellite communication system. What is also needed is a method and apparatus for mitigating the effects of satellite failure and/or satellite blockage in a satellite communication system in a cost-effective manner. What is further needed is an improved method and apparatus for maintaining service links in a satellite communication system.